The invention relates to electronic devices, and, more particularly, to radiation detectors and systems such as sensors which incorporate such detectors.
Detection of infrared radiation provides important approaches to night vision (imaging based on warm body emissions), chemical analysis (spectral absorption), and various other fields. Infrared detectors may be classified in various ways, such as single detector or pixel arrays, cryogenic (typically liquid nitrogen temperatures) or uncooled detectors, 8-12 mm or 3-5 mm or other wavelength sensitivity, and photon or thermal detection mechanism.
Photon detection (photoconductors, photodiodes, and photocapacitors) functions by photon absorption generating electron-hole pairs in small bandgap semiconductor materials; this increase in the number of electrical carriers is detected. In contrast, thermal detection functions by electrical resistivity or capacitance changes due to the heating of an element absorbing infrared photons. Detectors relying upon the change in resistivity due to photon heating are called bolometers.
Hornbeck, U.S. Pat. No. 5,021,663 and Keenan, U.S. Pat. No. 5,288,649 disclose an array of amorphous silicon bolometers suspended over and connected with CMOS control and drive circuitry in the form of a single semiconductor integrated circuit as could be used for night vision. In particular, FIG. 1a schematically illustrates lens system 102, array of bolometers 106, and circuitry for infrared imaging; FIG. 1b heuristically shows the circuitry of a single bolometer; and FIG. 1c shows a portion of an array of bolometers 140. Each bolometer provides the signal for a single pixel in a two-dimensional image. The bolometer suspension over the integrated circuit substrate provides thermal isolation but also engenders mechanical support problems. Bolometer packaging also presents problems because ambient atmosphere may provide thermal coupling of the bolometer with its surroundings and closely spaced detectors lead to crosstalk.
In FIG. 1b R.sub.B denotes the temperature variable resistance, R.sub.L a temperature independent load resistance, and +V a bias voltage applied across R.sub.B and R.sub.L in series for a single bolometer. The temperature variance of R.sub.B due to the varying infrared radiant power input during night vision applications typically is less than one degree Kelvin. The fluctuating temperature of R.sub.B implies a fluctuating resistance which induces a fluctuating voltage across load resistance R.sub.L, and this voltage drives the output amplifier. In general, the low frequency noise of the bolometer exceeds the Johnson noise associated with R.sub.B (white noise with amplitude proportional to the resistance) and increases in magnitude with the bias voltage applied across R.sub.B. Furthermore, the magnitude of the signal detected by R.sub.B -R.sub.L in series is proportional to the bias voltage. And often a bias sufficient to produce a measurable signal produces an unacceptable level of low frequency noise.
Infrared photoconductor detectors also typically have excessive low frequency noise. The usual approach to overcome this low frequency noise problem utilizes chopping (periodically mechanically blocking) the input radiation to measure the output for both irradiated and dark conditions, and then subtracting the dark condition output from the irradiated condition output to provide a net output ("correlated double sampling"). Such chopping greatly attenuates the effects of low frequency noise and improves the signal to noise ratio of the detector.
However, the chopped input approach has problems including the high-cost and low-reliability of mechanical systems. Further, thermal detectors such as bolometers require a substantial scene settling time in order to faithfuilly represent the signal level. For example, it is not uncommon for bolometers to require a signal interval of 30 milliseconds for faithful signal reproduction. Thus a maximum scene chopping frequency exits. But the effectiveness of correlated double sampling depends upon the scene chopping frequency being greater than the "1/f knee" frequency in the noise power spectrum of the detector. Thus mechanical chopping is not always an effective mechanism because the maximum scene chopping frequency due to scene settling time may be less than the 1/f knee frequency.
Bolometers and photoconductors may also detect visible light and near ultraviolet light and need not be limited to infrared applications; for example, colorimetry applications are just different wavelength applications.
Wong, U.S. Pat. No. 5,163,332 and Burough et al., U.S. Pat. No. 4,709,150 illustrate the use of infrared detectors to detect CO.sub.2 or other gases in the atmosphere by measuring absorption in a spectral line by the gas.